The OCT is a light coherence tomographic imaging method for inserting a light probe into an organ such as a blood vessel or an intestine of a patient, emitting low coherence light from the distal end of the light probe, and obtaining a precise tomographic image of the inside of a subject using light reflected in places inside the subject and returning to the light probe. A basic technique of the OCT is disclosed in Japanese Examined Patent Publication No. H6-35946 (Patent Document 1). A specific configuration of the light probe is disclosed in WO2011/074051 (Patent Document 2), Japanese Patent No. 4659137 (Patent Document 3), and the like.
FIGS. 8 and 9 show a conventional lateral light emitting device 11 (light probe) described in Patent Document 2. In the lateral light emitting device 11, a rod lens 3 is fused to one end of an optical fiber 2 and a prism 41 having a square cross section is fused to the distal end surface of the rod lens 3. As shown in FIG. 9, the prism 41 is inscribed in the rod lens 3. Note that reference sign 2a denotes coating of the optical fiber.
The lateral light emitting device of Patent Document 2 has a characteristic that, since the prism 41 is inscribed in the rod lens 3, the outside diameter of the lateral light emitting device is extremely thin and the lateral light emitting device can be inserted into an extremely thin blood vessel or the like and used. The outside diameter of the lateral light emitting device is considered desirably 250 μm or less. However, when a rod lens having a diameter of 200 microns is used, the outside diameter of the lateral light emitting device is preferably 200 μm.
In the lateral light emitting device of this type, the distance from an emission surface to a beam waist (a focal length) is required to be set long to a certain degree. To set the distance long, it is advantageous to set a beam diameter in a fused portion of the rod lens 3 and the prism 41 large. However, there is a problem in that, as shown in FIG. 9, if the beam 5 protrudes to the outer side of the prism 41, coupling efficiency is deteriorated and the performance of the lateral light emitting device falls.
In a lateral light emitting device 12 shown in FIGS. 10 and 11, the rod lens 3 is fused to one end of the optical fiber 2, a prism 42 having a square cross section is fused to the distal end surface of the rod lens 3, and the prism 42 is circumscribed with the rod lens 3 (FIG. 11)
In this case, in a fused portion of the rod lens 3 and the prism 42, a beam does not protrude to the outer side of the prism 41. However, there is a problem in that the largest diameter of the prism 42 (i.e., the outside diameter of the lateral light emitting device) increases. For example, when an outside diameter d of the rod lens 3 is set to 200 μm, the largest diameter D of the prism 42 is 282 μm, which exceeds 250 μm and undesirable.
FIGS. 12 to 14 are a lateral light emitting device 13 in which a prism lens 43 is fused to one end of the optical fiber 2. In the prism lens 43, the distal end surface of a GRIN lens (Graded Index lens) having a circular cross section is set as an inclined surface 43a inclined with respect to an axial line and a rear end surface is set as a connection surface connected to the optical fiber. In the lateral light emitting device, the outside diameter can be set extremely small and coupling efficiency is satisfactory.
As shown in FIG. 14, in the lateral light emitting device 13, since an emission surface of a beam is a curved surface, when media around the emission surface are substances greatly different from a circumference portion such as the air and water, the shape of an emission beam is formed in an excessively crushed elliptical shape, i.e., the emission beams is a so-called line beam. The lateral light emitting device 13 has a problem in that a beam waist distance is extremely short.
FIGS. 15 and 16 show a conventional lateral light emitting device 14 described in Patent Document 3. The lateral light emitting device 14 includes the optical fiber 2, the rod lens 3, one end of which is fused to the end surface of the optical fiber 2, and a prism 44 fused to the other end of the rod lens 3. The prism 44 has a base shape obtained by cutting a part of the circumference of a cylinder and forming a flat emission surface 44c parallel to an axial line. The prism 44 has a distal end inclined surface 44a formed by obliquely cutting the distal end part of the prism 44. Light entered in the prism from the optical fiber 2 is reflected on the distal end inclined surface 44a and emitted from the emission surface 44c. The rod lens 3 and the prism 44 are fused such that a center O1 of the rod lens 3 and a center O2 of a circular arc of the prism 44 coincide with each other.
In the lateral light emitting device 14, since the emission surface 44a is flat, a beam shape is substantially circular. The distance to the beam waist can be set long compared with the distance shown in FIGS. 15 and 16.
In FIGS. 15 and 16, the outside diameter of the rod lens 3 and the largest diameter of the prism 44 (the diameter of the circular arc) are equal. In this case, the outside diameter of the lateral light emitting device can be set extremely small. However, there is a problem in that, as shown in FIG. 16, the beam 5 protrudes to the outer side of the prism 44 in a fused portion of the rod lens 3 and the prism 44, coupling efficiency is deteriorated, and the performance of the lateral light emitting device falls.
To prevent the beam 5 from protruding to the outer side of the prism 44, as shown in FIG. 17, the outside diameter d of the rod lens has to be set considerably larger than the largest diameter D of the prism 44. For example, when the outside diameter d of the rod lens is set to 200 μm and width W of the emission surface 44c is set to 200 μm, the largest diameter D of the prism 44 is 282 μm, which exceeds 250 μm and undesirable.
Note that, in this case as well, the center O1 of the rod lens and the circular arc center O2 of the prism 44 coincide with each other.